It is already known to use light guides to illuminate panels for general lighting purposes and also for display applications (e.g. for illuminating signs and advertisements, and also for illuminating liquid crystal displays). In one form, often referred to as a light box, the light guide comprises a hollow box-shaped structure defining an optical cavity, and in another form it comprises a solid light-guiding plate. In both forms, a major surface of the guide can be illuminated by light directed into the guide in a direction generally parallel to that major surface, for example from at least one elongated light source or a similar arrangement located adjacent an edge of the light guide (so-called “edge-lit light guides”).
Illuminated panels based on edge-lit light guides are generally thinner than those that are lit from behind and, as a result, are visually attractive and also particularly useful when the depth of the space available for a panel is restricted. They also offer the advantage that the light source is separated to some extent from the panel so that the heat input into the latter from the light source is reduced. Hollow light guides would appear to offer further advantages for applications that require the weight of the light guide to be kept as low as possible but, despite that, solid light guides have typically been more widely used because they are comparatively simple to produce and are the easiest way of transporting light.
Light guides in the form of hollow light boxes are described, for example, in EP-A-0 490 279; 0 377 309; and 0 293 182; and in GB-A-2 310 525. In each of those light boxes, a prismatic optical film is employed with a view to achieving a more even distribution of light over the surface that is being illuminated. Practical designs for light boxes, intended for use in illuminating graphic displays, are described in an Application Bulletin entitled “Thin Light Box” and issued in March 1990 by Minnesota Mining and Manufacturing Company of St. Paul, Minn., USA. U.S. Pat. No. 6,080,467 describes an illuminated sign comprising a light box, the interior surfaces of which comprise a multi-layer reflective optical film.
An illuminated sign, suitable for use on an automotive vehicle, is described in WO 00/65277. The sign comprises a housing having diffusely-reflecting interior surfaces and a front sign face through which light from inside the housing is transmitted, the light being supplied by a light fibre located on an interior surface of the housing.
International patent application WO01/71248 describes a hollow light guide suitable for use in illuminating a graphic display. The front face of the light guide comprises Scotch™ Optical Lighting Film and forms a window through which light can leave the light guide. The rear face of the light guide comprises a highly-efficient specularly-reflecting optical film printed with an array of dots in a diffusely-reflecting ink. These dots form light-extraction elements and cause light to be emitted through the front face of the light guide. The arrangement of the dots on the rear face of the light guide is related to the size and shape of the light guide to yield a uniform illumination of the front face.
There is a continuing demand for improved illuminated panels especially, but not exclusively, for display purposes. One problem with many display panels is that the panel is more brightly illuminated in the area closest to the light source, which detracts from the overall visual appearance and effectiveness of the illumination. Accordingly, there is a demand for improved uniformity in the illumination and for the elimination, from the illuminated panel, of any visible signs of the location and nature of the light source(s). It is also highly desirable, from an environmental and a cost point of view, that the amount of power used for illumination purposes should be kept as low as possible.